Anime move errors

In the Pokémon anime, there will often come up a Pokémon that knows a move it cannot learn legitimately in any of the games.

It is unknown whether these are actual mistakes, a writing policy of not having to stick to the games too closely, or merely the Pokémon using an attribute that shares the name of a move in the games. In Bulbasaur... the Ambassador!, Professor Oak states that in times of need Pokémon can call upon power never known to it previously, such as Ash's Bulbasaur learning Dig.